Add ShadowOverScroller.

This adds a shadow for OverScroller, which makes it possible to test usages of DrawerLayout with behavior that depends on DrawerListener. Prior to this shadow existing, DrawerLayouts wouldn't really respond to being opened/closed, as their animations never ran (which they depend on to update the open/closed state).

After this patch, the animations will run as expected. (Note that you still need to get the DrawerLayout to recompute it's scroll state after the animation runs, e.g:
```java
Robolectric.getUiThreadScheduler().advanceToLastPostedRunnable(); // finish animation
drawerLayout.computeScroll(); // recompute scroll and open/closed state
```
)

Fixes #932.